1. Field of the Invention
The present invention relates generally to a device and method for encoding and decoding channel data in a mobile communication system, and in particular, to a device and method for encoding and decoding channel data using a turbo code.
2. Description of the Related Art
An encoder using a turbo code (hereinafter referred to as a turbo encoder) encodes an N-bit input frame into parity symbols using two simple parallel concatenated codes, wherein an RSC (Recursive Systematic Convolutional) code is generally used as a component code.
FIGS. 1 and 2 illustrate structures of conventional parallel turbo encoder and decoder, which are disclosed in U.S. Pat. No. 5,446,747 by Berrou, incorporated herein by reference.
FIG. 1 is a block diagram showing a configuration of a conventional turbo encoder. The turbo encoder of FIG. 1 includes a first constituent encoder 12, a second constituent encoder 14 and an interleaver 16 interconnected there between. For the first and second constituent encoders 12 and 14, an RSC encoder can be used, which is well known in the art. The interleaver 16 has the same size as a frame length of the input data (i.e., N bits), and decreases the correlation of the input data bitstream dk provided to the second constituent encoder 14. Therefore, the parallel concatenated codes for the input data bitstream, dk, become xk (i.e., dk without modification) and y1k, and y2k the outputs of the first 12 and second 14 constituent encoders.
FIG. 2 is a block diagram showing a configuration of a conventional turbo decoder. The turbo decoder includes an adder 18, subtracters 20 and 22, a soft decision circuit 24, delays 26, 28 and 30, and MAP (Maximum A Posterior Probability) decoders 32 and 34. The turbo decoder further includes an interleaver 36 which is identical to the interleaver 16 of FIG. 1, and deinterleavers 38 and 40. The turbo decoder repeatedly decodes data received by the frame unit using a MAP decoding algorithm, thereby decreasing a bit error rate (BER).
The utilization of interleaver 16 of the turbo encoder of FIG. 1 implies that encoding and decoding should be performed as a frame unit. Accordingly, it can be appreciated that the required memory and calculations required for the MAP decoders 32 and 34, shown in FIG. 2 are proportional to a value obtained by multiplying the frame size by a number of states of the first and second constituent encoders 12 and 14 of FIG. 1.
In a mobile communication system, voice and data are transmitted at a data rate of several Kbps to several Mbps, and a frame length of data input to a channel encoder may vary from several ms (milliseconds) to several hundred ms. For example, in the case where the data is transmitted at a data rate of over 32 Kbps, the number of data input to the turbo encoder is larger due to the high data rate, the turbo decoder requires more memory and calculations to decode the received data. The turbo encoder exhibits properties where an error correction performance is enhanced as the frame length of the input data becomes longer, however an increase in the memory and calculations is required in a decoder.
In addition, if the length of the input frame is too short, e.g., less than 8 kbps/10 ms, the interleaver 16 in the turbo encoder cannot sufficiently decrease the correlation among the input data, thereby deteriorating the error correction performance. That is, when the frame length of the input data is longer (or the input data rate is high), the turbo encoder structured as shown in FIG. 1 and the turbo decoder structured as shown in FIG. 2 require a lot of calculations and memory to perform encoding and decoding. Otherwise, when the frame length of the input data is shorter, the turbo encoder may exhibit lower performance results, as compared with a convolutional encoder or a concatenated encoder, thereby increasing the BER.
Accordingly, it is possible to decrease the required calculations and memory capacity required for decoding by appropriately varying the processing size of the data input to the turbo encoder, independent of the data rate for the corresponding service, while fully securing the low BER required in the communication system.